In a torque converter provided with a lock-up clutch, a lock-up clutch forward-reverse differential pressure (lock-up differential pressure) control engages and disengages the lock-up clutch. When the torque converter shifts from a converter state to the lock-up state, the lock-up differential pressure may gradually increase from a predetermined initial differential pressure. Namely, the torque converter shifts from a converter state to a lock-up state via a slip state. In the converter state, the lock-up clutch is not engaged, in the slip state, the lock-up clutch slips, and in the lock-up state, the lock-up clutch is completely engaged.
Tokkai Hei No. 2002-205576 published by the Japan Patent Office in 2004 discloses a control (lock-up control) of a lock-up clutch which performs lock-up from low speed and improves fuel consumption.